" Gasication phenomena of raw bamboo, torreed bamboo, and coal are studied." The carbon conversions of the three fuels are higher than 90%." The coal gas efciency is sensitive to the type of fuel." The gasication performance of torreed bamboo is enhanced and closer to that of coal." With optimum operation, syngas formation from torreed biomass is amplied by 88%.

a r t i c l e i n f o a b s t r a c t

Article history: Gasication of torreed biomass is a promising technique for producing synthesis gas (syngas) of higherReceived 14 September 2012 quality than has previously been available. In this study, in order to evaluate the potential of the tech-Received in revised form 21 December 2012 nique, gasication processes for three different materials, which include raw bamboo, torreed bambooAccepted 10 January 2013 (at 280 C for 1 h), and high-volatile bituminous coal in an entrained-ow gasier using O2 as the gasi-Available online xxxx cation agent, are studied numerically and compared to each other. The obtained results suggest that in all cases, the carbon conversions of the three fuels are higher than 90%. However, the cold gasication ef-Keywords: ciency for raw bamboo is low, mainly due to the relatively lower caloric value of the material. In the caseTorrefaction and gasicationEntrained-ow gasier of the torreed bamboo fuel, the gasication performance is enhanced signicantly and is quite similar toCarbon conversion the coal gasication under the same conditions. It appears that the optimum oxygen-to-fuel mass owCoal gas efciency ratios for the gasication of raw bamboo, torreed bamboo, and coal are 0.9, 0.7, and 0.7, and their equiv-Numerical simulation alence ratios are 0.692, 0.434, and 0.357, respectively. Under optimum conditions with respect to theClean coal technology equivalence ratio, the cold gas efciency of torreed bamboo is improved by 88%, as compared to raw bamboo. 2013 Elsevier Ltd. All rights reserved.

1. Introduction as an essential energy resource, development of a method by

which to utilize it in a cleaner way has become an important task. Fossil fuels are currently crucial sources of primary energy. It Over the years, the development of clean coal technology (CCT) hashas been reported that the reserves of oil and natural gas could last drawn much attention [2]. Gasication plays a key role in carryinganother 4060 years [1]. In contrast, the coal reserve is reported to out CCT where coals are converted into syngas (i.e. CO + H2), thebe available for over another 100 years, and its price is relatively main component in the product gas, in an insufcient oxygen envi-lower than that of oil and natural gas. Therefore, there is a trend ronment [3,4].toward the consumption of more coal instead of using oil as an en- In fact, not only coal but also other materials, such as biomass,ergy source. Over the past several decades, however, burning coal slurry, and petroleum coke, can be employed as feedstocks in gas-has caused numerous serious air pollution problems; it has also ication [57]. Biomass is an important source of renewable en-emitted a tremendous amount of carbon dioxide, especially from ergy in the world. For instance, according to statistics from thecoal-red power plants. For these reasons, while coal is consumed International Energy Agency (IEA) in 2011, around 10% of the pri- mary energy demand worldwide came from biomass. Biomass can be converted into gas products alone by gasication [8]; it Corresponding author. Tel./fax: +886 7 3740611. can also be gasied with coal through co-gasication [9,10]. Raw E-mail address: weihsinchen@gmail.com (W.-H. Chen). biomass is generally characterized by high moisture content and

low energy density as compared to those of coals. Typically, the thermodynamic loss, it has been seen as a promising method byhigher heating value of coal is in the range of 2535 MJ kg1 which to achieve more efcient gasication of wood. Deng et al.[11], whereas it is between 15 and 20 MJ kg1 for raw biomass [23] focused on the torrefaction of rice straw and rape stalk and[12]. This restricts the applications of raw biomass in industry. found that the heating values of torreed rice straw and rape stalk To use biomass more efciently, a variety of pretreatment could be increased up to 17% and 15%, respectively. They also camemethods for improving biomass have been developed. Torrefaction up with a conceptual system by combining coal gasication andis one of the most noteworthy routes. It is a thermal pretreatment biomass torrefaction. Couhert et al. [24] investigated the impactprocess where raw biomass is heated in an oxygen-free environ- of torrefaction on the production of syngas from wood gasicationment at temperatures of 200300 C [1316] in order to perform in an entrained-ow reactor. They addressed the fact that charsthermal degradation while avoiding oxidation of the biomass. from torreed wood were less than those from wood, and theThe moisture contained in the biomass is reduced, and the compo- quantity of produced syngas increased with the severity ofnents of low-molecule volatiles are released as a result of torrefac- torrefaction. Strege et al. [25] used blends of subbituminous coal,tion. As a consequence, the properties of the biomass are improved torreed biomass, and untreated biomass to study their gasica-to a great extent. The advantages of torrefaction of biomass include tion phenomena in a bench-scale oxygen-blown uid-bed gasierintensifying caloric value and energy density [17,18], reducing O/ that was coupled to a modular xed-bed FischerTropsch reactor.C and H/C ratios [19], producing hygroscopic materials, and mak- Their results indicated that the bed temperature diverged rapidlying the grindability and storage of biomass easier [20]. Moreover, and material agglomeration occurred in the bed when a blend ofthe properties of torreed biomass become more uniform com- coal and torreed biomass was replaced by a blend of coal andpared to those of raw biomass [21]. raw biomass; therefore, using torreed biomass as the feedstock On account of property improvement of biomass from torrefac- of gasication was found to be conducive to minimizing thetion, this pretreatment method is conducive to the performance of formation of agglomerates.gasication if torreed biomass rather than raw biomass is used as In examining the literature published recently on this topic, afeedstock. In the study of Prins et al. [22], it was pointed out that number of studies of biomass torrefaction have been implemented,the gasication temperature of raw wood was lower than that of and much valuable information has been reported. However, verytorreed wood due to the high O/C ratio and high moisture content few papers have highlighted the gasication of torreed biomass,in the former. As a result, wood was generally over-oxidized in a and there still remains a lot of knowledge that is not availablegasier, which led to thermodynamic loss. Because torrefaction in sufcient detail. Therefore, the objective of this study is tocan decrease moisture and O/C ratios and thereby reduced investigate the reaction behavior of torreed biomass. Particular

emphasis is placed on the comparison of gasication characteris- drags coupled to mass, energy, and momentum transport from thetics among coal, raw biomass and torreed biomass. particles into the uid. A discrete-phase-model (DPM) is employed to track the trajectories of fuel particles through the continuous2. Numerical method phase of uid. In the model, the velocity change of a particle is written as2.1. Geometry of reactor mp dV P =dt F d 1 Fuel gasication in an entrained-ow gasier is explored in the where VP is the particle velocity, and Fd is the drag force of the uidpresent study, and the geometry of the gasier is sketched in on the particle. The drag force Fd is expressed asFig. 1a. The gasier was a cylindrical, dry feed, and pressurized en-trained-ow reactor [7], and its height and diameter were 5151.2 qAp;c C D v 2rand 270.3 mm, respectively. The reactants and carrier gas were FD 2 2blown into the gasier from the top of the reactor. Fuel and carriergas were sent into the gasier from the center inlet, whereas oxy- where q, AP,c, and vr are the density of the surrounding uid, thegen was transported from the concentric ring inlet. The distance cross-sectional area on the particle normal to the direction of thefrom the oxygen inlet to the gasier centerline was 45 mm. The ow, and the relative velocity between the particle and the uid,diameter of the center inlet and the width of the concentric ring in- respectively. CD is the drag coefcient, and it is a function of the par-let were 20 and 5 mm, respectively. ticle Reynolds number [26]. Several heat and mass processes of fuel particles take place in2.2. Assumptions and governing equations the gasication reactions, so they are considered in the source terms of the governing equations. To begin with, the moisture in Fuel reactions in a gasier are related to uid dynamics, heat and the fuel particles is evaporated; then, the particles undergo devol-mass transfers, and chemical reactions. To make the physical prob- atilization processes; furthermore, the particles are converted intolems involved in these processes more tractable, some hypotheses volatiles, chars, and ash; nally, the volatiles and chars are con-are adopted. (1) A steady, axisymmetric, incompressible, and sumed to produce gases [27]. The consumption rate of species iturbulent ow eld is adopted. (2) Thermal radiation and the body at the particle surface (Ri ) is presented byforce of the ow in the gasier are ignored. (3) The formation of Ri Ap byi r i 3air pollutants from gasication, such as NH3, HCN, H2S, CS2, andCOS, are disregarded. (4) The wall of the gasier is adiabatic. 0 r i NAccordingly, a two-dimensional and steady ow with turbulence ri ki pn 4 Dand chemical reactions is studied numerically, and the steady-statetime-averaged NavierStokes, energy, and species equations are In the preceding two equations, Ap is the surface area of the particle;solved. The standard ke turbulence model is also adopted. The b is the effectiveness factor; yi is the mass fraction; r is the per unit 0aforementioned governing equations and the constants used in the area species reaction rate; ki is the reaction rate constant; pn is thestandard ke turbulence model are tabulated in Table 1. partial bulk pressure of the gas phase species; D is the diffusion coefcient, and N is the apparent order of reaction. The reaction rate 02.3. Discrete phase model constant ki follows the Arrhenius equation, and the apparent order of the reaction depends on the oxygen concentration [28]. For the The behavior of fuel particles in uid is modeled by an Eulerian order of unity (N = 1), the consumption rate of species i at the par-Lagrangian approach that considers the inertia and hydrodynamic ticle surface is given by

Fig. 1. Schematics of (a) geometry and (b) grid system of the investigated entrained-ow gasier.

In the above equation, the forward reaction rate constant kf is estab- _ out yH2 HHVH2 yCO HHVCO yCH4 HHVCH4 mlished based on the Arrhenius law, where A is the pre-exponential CGE % _ in;fuel HHVfuel mfactor, and B is the temperature exponent. Their values in various 100 29chemical reactions are listed in Table 2 as well. where yi is the mass fraction of species i in the product gas. That is,2.5. Boundary conditions CC is evaluated from the concentrations of CO2, CO, and CH4 in the product gas, whereas CGE is determined from the concentrations of The fuel at 300 K was fed into the gasier from the top of the H2, CO, and CH4.reactor, and a carrier gas (i.e. air) at 300 K was employed to aidin the transporting of the fuel particles. Their mass ow rates were 3. Results and discussioncontrolled at 0.023 and 0.025 kg s1, respectively. The sizes of thefuel particles were in the range of 44250 lm, and the average Three different fuels, comprised of raw bamboo, torreed bam-particle size was 103 lm. This was determined using the boo [38], and high-volatile bituminous coal [39], serve as the basisRosinRammler distribution function [34]. Oxygen was used as of this study. The torreed bamboo was produced from the torre-the oxidant. The centerline of the gasier was an axisymmetric line faction of raw bamboo at 280 C for 1 h followed by natural coolingso that no heat and mass uxes could pass through the line. The to ambient temperature. The properties of the fuels, such as prox-wall of the gasier was conceived as an adiabatic wall, and no slip imate, elemental, and caloric analyses, are summarized in Table 3.conditions were obeyed. At the outlet, the pressure was xed at In this study, the O/F ratio was controlled between 0.5 and 1.1 by2 MPa, implying that the value was the operating pressure. varying the mass ow rate of oxygen. According to the O/F ratio, the ER was in the range of 0.2250.849.2.6. Numerical method and validation

The commercial software ANSYS FLUENT V12 was used in this 3.1. Gasication phenomenastudy to simulate coal and biomass gasication phenomena. Inthe simulations, the SIMPLE algorithm was utilized to solve the Fig. 2 rst demonstrates the isothermal contours of the threegoverning equations in association with the boundary conditions. fuels in the gasier, where the mass ow rates of fuel and air (car-A rst-order upwind scheme was employed to calculate the con- rier gas) are 0.023 and 0.025 kg s1, respectively and the O/F ratiovection and diffusion uxes. A 20 1000 (viz., radial axial) grid is 0.9. Under the condition of O/F = 0.9, the mass ow rate of oxy-system was adopted for the predictions in that the system satised gen is 0.01545 kg s1. The gure shows that the high-temperaturethe requirement of grid independence [35]. Solid tests for the val- zone is always located in the vicinity of the fuel and oxygen inlets,idation of chemical kinetics (Table 2) have been carried out in a regardless of which fuel is consumed. This is attributed to the mix-previous study [35], where the numerical predictions were in good ing of fuel and oxidizer followed by the occurrence of exothermicagreement with the experimental results. combustion (i.e. Eqs. (13), (16), (17), and (19)). The temperature decreases downward as a consequence of the endothermic gasi-2.7. Parameters and performance indices cation reactions (i.e. Eqs. (14), (15), and (20)). The maximum tem- perature in the gasier depends highly on the type of fuel. It can be The oxygen-to-fuel (mass ow rate) ratio (O/F ratio) and equiv- seen that relatively more volatile matter (=80.13 wt.%) and lessalence ratio (ER) are two important parameters in practicing fuelgasication. The ER is dened as the following [36]: Table 3 m_ oxygen =m_ fuel real Proximate, elemental, and caloric analyses of fuels.ER 24 _ oxygen =m m _ fuel stoichiometric Raw bamboo Torreed bamboo Coal (at 280 C for 1 h)The stoichiometric mass ow ratio of oxygen to fuel, _ oxygen =m m _ fuel stoichiometric , can be obtained in accordance with the Proximate analysis (wt.%, dry basis) VM 80.13 58.80 34.86elemental analysis and the law of conservation of mass or atoms. FC 17.75 37.78 58.92When determining ER, it is assumed that the fuel is comprised of Ash 2.12 3.42 6.22carbon, hydrogen, and oxygen, and nitrogen is ignored due to its Elemental analysis (wt.%, dry basis)low content. When the three fuels react with oxygen stoichiometri- C 46.78 58.43 67.71cally, they are individually expressed as follows: H 6.38 5.10 4.95 N 0.25 0.34 1.18Raw bamboo CH1:635 O0:743 1:03725O2 O 46.59 36.13 26.16

xed carbon (=17.75 wt.%) are contained in the raw bamboo the gasier (Fig. 4b). However, it is noted that more CO2 is pro-(Table 3); meanwhile, its O/C and H/C ratios are higher. duced in contrast to the raw bamboo. With regard to coal gasica-Consequently, the higher heating value (HHV) of raw bamboo is tion, the concentrations of H2O and CO2 at the exit are the lowestlower (=18.75 MJ kg1), and the maximum temperature in the gas- among the three fuels. A comparison between the gasication ofier is 1348 K (Fig. 2a). The HHV of the torreed bamboo is shown raw bamboo and torreed bamboo reects that the latter is closerto be 22.50 MJ kg1, so more heat is released from its burning com- to that of coal. It is thus pointed out that the improved propertiespared to that of the raw bamboo. This is the reason that the max- of biomass from torrefaction make the gasication of biomass ap-imum temperature of the torreed bamboo reaches 2268 K proach that of coal. The concentration of CH4 is extremely low(Fig. 2b). The HHV of coal is shown to be 26.22 MJ kg1, which is (<0.01%) in all cases. Hence, the concentration contours of methanemuch higher than that of the raw bamboo and the torreed are not presented. Figs. 24 indicate that the temperature and spe-bamboo. Hence, when the coal reacts with oxygen in the gasier, cies concentrations are almost invariant adjacent to the exit of thethe highest temperature is 2840 K (Fig. 2c). Corresponding to the reactor, implying that fully developed fuel gasication has beengasication of the raw bamboo, torreed bamboo, and coal, the accomplished. The designed geometry of the reactor allows theaverage temperatures at the exit of the gasier are shown to be implementation of fuel gasication.437, 935, and 1002 K, respectively. This reveals that the role of heatsink played by endothermic gasication (i.e. Eqs. (14), (15), and(20)) has a profound effect on temperature distribution. 3.2. Syngas formation and gasication performance In examining the concentration contours of CO and H2 shown inFig. 3, the gure depicts that the concentrations of CO and H2 from Fig. 5 displays the proles of mole fractions of H2, CO, and syn-the gasication of the raw bamboo are relatively low, especially in gas (i.e. H2 + CO) in the product gas under the gasication of thethe case of H2 (Fig. 3a). When the torreed bamboo is gasied, the three fuels where the O/F ratio is 0.9. For the raw bamboo, mostformation of H2 is intensied markedly compared to that of the atomic hydrogen is transformed to H2O (Fig. 4a), so the concentra-raw bamboo, whereas the CO formation increases slightly tion of H2 is very low. On account of insufcient oxygen supplied in(Fig. 3b). By virtue of the higher HHV and larger amount of carbon an environment of gasication, some CO is generated. The molecontained in the coal, the CO concentration from coal gasication is fractions of CO and syngas (i.e. CO + H2) in the product gas arefurther enlarged (Fig. 3c). The distributions of H2O and CO2 are dis- around 0.162 and 0.175, respectively. When the torreed bambooplayed in Fig. 4. As a result of the maximum temperature of the is gasied, the H2 formation is raised signicantly, whereas the for-raw bamboo gasication being lower (Fig. 2a), it is relatively dif- mation of CO is close to that of the raw bamboo. The mole fractioncult to complete the endothermic reduction reactions (i.e. Eqs. (14), of syngas in the product gas is 0.364, accounting for a 109% incre-(15), and (20)). As a consequence, the concentrations of H2O and ment in contrast to that of the raw bamboo. This results clearlyCO2 from the gasication of the raw bamboo are shown to be high- indicate that torrefaction is able to facilitate syngas formation fromer (Fig. 4a). The gasication of the torreed bamboo is conducive to biomass gasication, and this behavior is qualitatively consistentthe formation of H2 (Fig. 3b), so less H2O is retained at the exit of with the results of Couhert et al. [24]. With regard to coal

gasication, the mole fraction of syngas is found to be 0.574, stem- in these fuels was converted to CO, CO2, and CH4. Unlike CC, theming from the increase in the CO generated. cold gas efciency (CGE) is sensitive to the type of feedstock. Spe- Upon inspection of the carbon conversion (CC) shown in Fig. 6, cically, the values of CGE from the gasication of the raw bamboo,the values for the raw bamboo, torreed bamboo, and coal are 92.2, torreed bamboo, and coal are 29.0, 49.8, and 69.1%, respectively.91.3, and 98.5, respectively, revealing that over 90% of the carbon The CGE of the bamboo is amplied by a factor of 1.72

Fig. 6. Proles of carbon conversion and cold gas efciency for different fuels(O/F = 0.9). Obviously, the outlet mean temperatures rise with increasing O/F ratios, regardless of which fuel is consumed (Fig. 7a). This arises(=(49.8% 29%)/29%) after experiencing torrefaction. The value of from the fact that the exothermic oxidation reactions are intensiedthe CGE is determined from the heating values and mass fractions as the O/F ratio increases. It is noted that the difference in outletof CO, H2, and CH4. The high CC and low CGE resulting from raw mean temperature between the raw bamboo and torreed bamboobiomass gasication are attributed to the high concentrations of becomes pronounced (>500 K) as the O/F ratio becomes larger thanCO2 and H2O as well as to the low concentrations of CO and H2 or equal to 0.9. This reects that the O/F ratio has a pronouncedin the product gas. Accordingly, it should be pointed out that a high inuence on the thermal behavior of gasication at O/F = 0.9. TheCC value does not with certainty lead to better gasication perfor- higher mean temperature resulting from the reactions of the torr-mance. The difference in the CGE between the raw biomass and the eed bamboo is conducive to its gasication. Consequently, the syn-coal is around 40%. Nevertheless, the CGE of the bamboo is in- gas formation from the torreed bamboo is much higher than thatcreased by 21% (=49.8% 29%) from the torrefaction process, as a of raw bamboo (Fig. 7b). Unlike the outlet mean temperature, theresult of more H2 being produced (Fig. 5). These results qualita- maximum syngas formations of raw bamboo, torreed bamboo,tively agree with the ndings of Deng et al. [23]. and coal take place at O/F = 0.9, 0.8, and 0.7, respectively. The proles of the gas concentrations of CO, H2, CO2, and H2O at3.3. The effect of O/F ratio on gasication various O/F ratios are plotted in Fig. 8. In the case of the raw bam- boo gasication, it can be seen that the concentrations of CO2 and The proles of the mean temperatures at the exit of the gasier H2O are higher than those of CO and H2 within the investigatedfrom the gasication of the three fuels and the mole fraction of range of O/F ratios (Fig. 8a). This can be explained by higher atomicsyngas at various O/F ratios (0.51.1) are presented in Fig. 7. The O/C and H/C ratios in raw biomass which facilitate the formationsoutlet mean temperature is obtained by of CO2 and H2O. The O/C and H/C ratios are reduced notably when R the bamboo is torreed (Table 3). The formations of CO and H2 are AR q T v dAT out 30 thus enlarged to a certain extent (Fig. 8b). Again, this means that A q v dA torrefaction can improve the gasication performance of biomass.

(a) Bamboo 3.4. Gasication performance at various equivalence ratios

0.6 CO2 The O/F ratio merely provides an indication of oxygen supply. In H 2O 0.5 H2 view of the different amounts of C, H, and O contained in biomass CO and coal (Table 3), the equivalence ratio is a more feasible indicator by which to describe oxidant supply in terms of the nature of fuel. 0.4 The distributions of the CC for the three fuels at various equiva- Mole fraction

lence ratios are shown in Fig. 9. It is not surprising that an increase

0.3 in ER leads to the growth of the CC, resulting from more oxygen supplied for reactions. By virtue of the lower HHV for raw bamboo, 0.2 a higher ER is required to reach a higher CC. Alternatively, coal is featured by a higher HHV; more heat is liberated when it is oxi- dized. As a consequence, a higher CC can be obtained at a relatively 0.1 lower ER compared to that of raw bamboo. As a whole, whether raw biomass, torreed biomass, or coal is used for gasication, a 0 CC of over 97% can be achieved if the value of the ER is high 0.5 0.6 0.7 0.8 0.9 1 1.1 enough. O/Fratio Instead of the monotonic distributions developed in the CC, the CGE curves of the three materials are characterized by a maximum (b) Torrefied bamboo distribution, as shown in Fig. 10. This characteristic is consistent 0.6 CO2 H 2O 0.5 H2 CO 100 0.4 Mole fraction